Conventional passively Q-switched microlasers were disclosed in U.S. Pat. No. 5,394,413 issued Feb. 28, 1995 to Zayhowski.
The pumping of microlasers by fiber-coupled pump diodes has also been widely disclosed in the prior art, including in the afore-mentioned patent.
Amplification of passively Q-switched microchip lasers was disclosed in U.S. Pat. No. 5,909,306, issued Jun. 1, 1999 to Goldberg et al., as well as by Di Teodoro et al. in 2002 (Optics Letters, 27, 518, 2002). However, these devices were based on standard microlasers with bulk packaging without fiber coupling, resulting in a large and cumbersome product. In particular, they do not capitalize on the thermal management and ruggedness of a fiberized configuration.
Several techniques for packaging passively Q-switched microlasers have been reported; for example, Holm et al. (U.S. Pat. No. 6,754,418) reported the use of a silicon microbench in order to reduce the size and cost, while providing good thermal management. However, no corresponding packaging or assembly process has been developed, so that no breakthrough was obtained in the product performance.
MegaWatt Lasers Inc. (Trussel et al., Lase 2004, presentation 5332-14) reported the packaging of passively Q-switched microchip lasers in a TO-3 package for eyesafe emission. However operation was limited to a very low repetition rate (<10 Hz). Moreover, the assembly process relied on manual dynamic alignment with subsequent gluing, and therefore was not suitable for volume production and cost reduction. The TO-3 package also did not afford output coupling through a fiber.
Pumping microchip lasers out of the absorption peak has been reported by Trussel et al. (e.g. eye-safe Yb:Er:glass, Lase 2004, presentation 5332-14). The insensitivity of the absorption coefficient to the pump wavelength allows the product to operate without temperature control. However, for a given microchip, non-optimized pumping conditions lead to very limited product performance.
While coaxial packages and associated assembly processes have been developed for fiberoptic communication components such as WDM, filters, attenuators, to date no attempt has been made to apply them to solid-state lasers.
A laser diode pumped solid state laser with miniaturized quick disconnect laser head, disclosed in U.S. Pat. No. 4,665,529, issued May 12, 1987 to Baer et al, comprises an extended cavity with a large number of mechanical parts and complex geometries, which do not favor manufacturing cost reduction. The mechanical parts also do not offer the degrees of freedom for adjusting the focus of the pump diode or for adjusting the relative lateral position of the different optical parts. No provision has been made for coupling into an output fiber, so there is no opportunity to build a versatile product platform on this technology. The quick-disconnect fiber connector also provides poor positional accuracy, which could be detrimental to the performance of a passively Q-switched microchip laser, where output performance parameters are directly related to the pump intensity. As a consequence, the disclosed design may not be directly applied to pulsed microchip lasers.
A continuation of the same idea is disclosed in U.S. Pat. No. 4,723,257, issued Feb. 2, 1988 to Baer et al, including the possibility of using monolithic laser cavities, removing the focusing optics, using the quick-disconnect connector on the pump diode side, and using arrays of pump diodes or broad area pump diodes. While this arrangement may be satisfactory in C.W. lasers, it is not directly applicable to pulsed microchip lasers.
In the case of a microchip laser cavity, the laser mode is no longer fixed by the cavity geometry but only depends on the pump beam focusing. The accurate control of the position of the different elements (input fiber, focusing optics, microlaser) is then critical for the stability and reproducibility of the laser performance. From this point of view, the present invention offers important advantages compared to those disclosed in the above two patents.
U.S. Pat. No. 6,282,227 issued Aug. 28, 2001 to Schmitt et al, discloses a diode-pumped frequency doubled solid-state laser operating in C.W. with intra-cavity frequency doubling. Schmitt discloses the use of a monolithic metallic part to improve the mechanical stability and thermal management, which is not directed toward (and not compatible with) the reduction of cost and size of the laser head.
In U.S. Pat. 6,434,177, issued Aug. 13, 2002 to Jurgensen, a solid-state laser with one or several pump light sources is disclosed, wherein a specific optical part is used to increase the pump power of a solid state laser by combining several pump diodes (or arrays). Jurgensen discloses corresponding coupling schemes. The cylindrical package disclosed is monolithic, so that it offers no degree of freedom for adjusting the relative position of the optical parts (focusing, resonator). It should also be noted that it is limited to C.W. lasers with extended cavities, and could not be applied to microchip lasers for all the reasons described before (see U.S. Pat. No. 4,665,529 for example). In addition, output fiber coupling is not considered.
Further, a solid-state laser device based on a tubular housing containing a passively Q-switched microchip laser for generating the fourth harmonics has been described in “Conference on Lasers and Electro-optics (CLEO), p.236, paper CWA6, 1996”. In this conventional solid-state laser device, an optical fiber is connected to a Nd:YAG laser crystal, a Cr:YAG saturable absorber crystal as Q switch element and a KTP crystal.
While the arrangement of optical elements in the cylindrical housing is quasi-monolithic, no degrees of freedom are provided for alignment of the optical parts. No focusing optics is included for the pump beam, and no fine adjustment of the fiber end position is provided. Accordingly, as described previously, the stability and reproducibility of the laser performance will be considerably limited. As no output fiber coupling is provided, there is little opportunity to build a versatile product platform based on the disclosed technology. Also, optical adhesives are used for the assembly of the optical parts. In the case where ultraviolet laser radiation is generated, deposition of organic contamination under UV light may limit the reliability of the laser, because all optical parts are exposed, being located in the same housing.
U.S. Pat. 6,456,637, issued on Sep. 24, 2002 to Holcomb, discloses the use of two pump wavelengths, from each side of the absorption peak, in order to improve thermal stability of the laser or amplifier. This embodiment relies on a very accurate choice of the pump wavelengths, which in turn requires accurate sorting of the pump diodes. A lower yield in the process will result due to the higher number of rejected parts, thereby increasing the manufacturing cost. In contrast, an aspect of the present invention is based on using a pumping wavelength far from the absorption peak, in a spectral range where the absorption is quasi-insensitive to the pump wavelength, so that no sorting of the pump diodes is needed. As a consequence, the combination of optimized pumping efficiency and off-resonance pump wavelength helps achieve acceptable laser performance over a wide range of temperatures.
Another disclosure of off-peak optical pumping appears in U.S. Patent Application 20040101015A1, published Sep. 8, 2005 by Lefort et al., which is directed to reducing thermal stresses in Nd:YVO4 (yttrium orthovanadate) crystals or rods as a result of end pumping with light at the peak-absorption wavelength of about 808 nm. Such stresses can fracture the laser crystal under strong pumping. More specifically, the pump wavelength is chosen so that the absorption is quasi-isotropic, which is not the case at resonance where the strong anisotropy leads to uniaxial stress and cleavage. However, no reference is made to the temperature dependence of the laser performance. The result is achieved by optically end-pumping at a wavelength at which the absorption is less than about 35% of the absorption at 808 nm, a preferred wavelength range being between about 814 and 825 nm.
A diode pumped Nd:YAG laser is disclosed in U.S. Pat. No. 4,734,912, issued Mar. 29, 1988 to Scerbak, wherein a YAG rod optical resonator coated on its ends is made sufficiently short, i.e., 1 mm, so that it will support only two axial resonant (lasing) modes. The rod is transversely stressed to polarize the two original modes and to excite a third lasing mode orthogonally polarized to the first two modes. The third mode is separated from the first two modes to provide stable, single mode TEM001 output. The transverse stress is applied by means of a spring clamp made of a material, Be—Cu, having a low temperature coefficient.
In the case of U.S. Pat. No. 4,953,166, issued Aug. 28, 1990 to Mooradian, mechanical stress is applied to tune the short resonant cavity having a free spectral range larger than the gain bandwidth, for example thermally or by the application of a longitudinal or transverse stress. The position of the longitudinal mode with respect to the gain curve is adjusted to control all output performance.
In summary, no disclosures have been in made in prior art for a low-cost, high reliability, high performance platform for manufacturing passively Q-switched lasers which are directly compatible with existing optical telecommunication fiber-coupled components.
An object of the present invention is to overcome the shortcomings of the prior art by providing a solid state laser platform wherein there is a reduction of size, reduction of power consumption, broadening of operating temperature range, reduction of product cost, including reduction of labor hours (current processes are based on manual assembly with active alignment), availability of output through a fiber, amplification of pulses (either in bulk or in fiber amplifiers) to increase output power, generation of new wavelengths (with frequency conversion crystals) and add-on modules (controllable attenuators or switches, monitor photodiodes, etc.).
Most of these issues are related to the laser packaging. A smaller size leads to lower passive heat load, reduced power consumption for thermal management and/or a broader operating range; a package with fiber output also permits the use of a fiber amplifier, with the associated higher output power facilitating the generation of new wavelengths; and a suitable design of the package that helps to leverage existing assembly processes for cost reduction.